1. Field of the Invention
The present invention relates to an electronic magneto ignition for internal combustion engines which have a permanent magnet mounted on the engine fly wheel and which have a magnetic field coil with a primary winding and a secondary winding in which ignition current and voltage, respectively, is produced during the passage of the permanent magnet, and more particularly to such an ignition in which a switching transistor is connected in parallel with the primary winding of the magnetic field coil, in which a diode is connected in parallel to the primary winding of the magnetic field coil, and in which an electronic circuit blocks the switching transistor at the desired ignition time so that the primary circuit is interrupted and the high voltage for ignition of the fuel mixture is produced in the secondary winding.
2. Description of the Prior Art
Magneto ignitions, in contrast to battery ignitions, are not fed by an accumulator. Instead, the required energy is produced in a coil which is generally identical with the ignition coil, by means of a permanent magnet moving past the coil. The electric values of the induced signals are dependent on the angle and the speed of rotation. The ignition must result from an irregular interruption of the ignition coil circuit.
The permanent magnet is mounted on the fly wheel of the engine. If the engine moves the permanent magnet past an ignition coil which is mounted on a U-shaped magnet core, and if the breaker points are closed, then a small negative current flows at first when the magnet enters the coil field. If both the magnetic poles are located in the magneto circuit of the coil, then the current flows in the positive direction. The greatest instantaneous current flows if the magnet is centrally located across the coil. If the breaker points are now opened with the aid of a cam, if possible during the greatest current flow, then a high voltage is formed in the coil by means of the sudden current interruption. This voltage can be stepped up with the aid of a secondary winding so that voltages up to 30 kV are formed which are conveyed to the spark plug to trigger the ignition spark for the combustion of the fuel mixture.
Magneto ignitions which have mechanical contacts, still widespread in use at the present time, have decisive disadvantages; they are, in particular, the heavy burning of the contacts, the spark advance produced by this burning whereby not only the engine output can be lowered but also the ignition time can be situated beyond the maximum current flow, and the contact openings and closings which make the use of mechanical contacts impossible at rotational speeds above 8000 rpm. For these reasons, electronic solutions are being increasingly employed, in spite of the higher costs involved.
A transistor is offered as an electronic switch having good cut-off properties, in contrast to the thyristor. However, the production of the base control current is a problem in the use of a transistor. A conventional solution utilizes a switching transistor in parallel with the primary winding of the ignition coil.
A control transistor is connected in parallel to the base-emitter path of the switching transistor and a Zener diode is connected to the base of the control transistor. If the coil is excited, the control transistor is presently blocked and the switching transistor becomes conductive by way of a base resistor. The primary current therefore flows by way of the switching transistor wherein, however, the residual voltage drop remaining in the switching transistor cannot be neglected. This type of known circuit is dimensioned in such a manner that the transistor residual voltage increases close to the current maximum, so that the control transistor receives control current by way of the Zener diode and becomes conductive. At this moment, the switching transistor is blocked, and the primary circuit of the ignition coil is interrupted, whereby the ignition voltage is formed. The disadvantage of this known type of circuit resides in the fact that in the primary circuit a high residual voltage must remain in the transistors and base resistances, in order to produce the control current, and in that an ignition time adjustment dependent upon the rotational speed is not possible.